1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a device manufacturing method and relates, for example, to a reflecting type projection optical system that is ideal for x-ray projection exposure apparatus that uses x-ray to transfer a circuit pattern on a mask onto a photosensitive substrate by means of a mirror projection system.
2. Description of the Related Art
Exposure apparatus that uses x-ray has gained attention as exposure apparatus used in the fabrication of semiconductor devices, etc. If x-ray is used as the exposure light, the usable transmitting optical materials and refracting optical materials will no longer be present, so, in addition to using a reflecting type mask, a reflecting type projection optical system is used. In the past, 8-mirror reflecting type optical systems consisting of eight reflecting mirrors have been proposed as projection optical systems that can be applied to exposure apparatus that uses x-ray as the exposure light, for example, in the specification of U.S. Pat. No. 6,710,917 (corresponds to Japanese Unexamined Patent Application Publication No. 2002-139672).
The conventional reflecting type projection optical system disclosed in the second working example of U.S. Pat. No. 6,710,917 is a 3 times image forming type optical system that forms intermediate images between a second reflecting mirror and a third reflecting mirror and between a sixth reflecting mirror and a seventh reflecting mirror respectively. In this example, the sixth reflecting mirror is a reflecting mirror that has a reflecting region (usage region) at the position most separated from the optical axis, that is, the reflecting mirror with the largest effective radius. The reflecting surface of this largest sixth reflecting mirror is formed into a spherical shape, so regardless of the fact that the reflecting region is at a position that is relatively greatly separated from the optical axis, it is relatively easy to detect the surface shape of the reflecting surface of the sixth reflecting mirror using an interferometer. In contrast with this, it is not easy to use an interferometer to detect the surface shape of a reflecting surface with an aspheric surface shape at a position that is relatively greatly separated from the optical axis.
However, in the conventional reflecting type projection optical system discussed above, while the maximum object height H0 is 110 mm, the effective radius Mφ of the sixth reflecting mirror, which is the largest, is as much as approximately 400 mm, so the ratio Mφ/H0 of the maximum effective radius Mφ to the maximum object height H0 is approximately 3.66, resulting in an optical system that has become extremely large in the radial direction. In addition, the total length (the object-to-image distance) TT of the optical system is approximately as much as 1956 mm, so the ratio TT/H0 of the total length TT to the maximum object height H0 is approximately 17.8, resulting in an optical system that has become extremely large in the axial direction. In addition, in the conventional reflecting projection optical system discussed above, the image side numerical aperture NA is 0.4, but there are demands to further increase the image side numerical aperture NA to achieve high resolution.